This research seeks to identify and characterize the hierarchy of regulatory phenomena which govern the expression of a simple metabolic pathway a eukaryotic organism. The pathway is that of nitrate assimilation in Neurospora crassa, which consists of two pyridine nucleotide-dependent electron-transfer enzymes: nitrate reductase and nitrite reductase. Both enzymes of this pathway are adaptively formed in the presence of either nitrate or nitrite, and both are repressed by the end product, ammonium. This latter effect is dominant. Further, rapid inactivation of the nitrate reductase occurs in vivo in the presence of ammonium or in the absence of any nitrogen or carbon source. Immunoelectrophoretic procedures have been employed to provide a highly sensitive and specific assay for nitrate reductase protein that is independent of its expression of any catalytic capability. These techniques permit the determination of nitrate reductase levels under varying nutritional regimens which influence its synthesis and decay and afford the opportunity for observing the regulation of nitrate reductase gene expression in wild-type N. crassa and in mutants devoid of enzymatic function. Related investigations include a definitive mutational analysis of the genetics of nitrate assimilation to categorize all loci involved and characterize their phentoypic behavior at the nutritional and biochemical level. These results will permit research focused on the elucidation of the regulatory gene action which manifests the ultimate control over expression of the metabolic potentiality inherent in this mitrate assimilatory pathway.